Polyamide fibers have good toughness, adhesion, fatigue resistance and other properties, and are widely used as industrial materials. Of polyamide fibers, a polyhexamethylene adipamide fiber is especially suitable for products which are used under severe conditions or for which a high quality is required. Excellent dimensional stability to high temperature and thermal resistance of this fiber is utilized in the step of processing the fiber for the manufacture of the products.
It is always required that industrial products are light-weight and thus it is important that the amount of reinforcing fibers contained in the industrial products is minimized without substantial reduction of the reinforcing performance. For satisfying this requirement, fibers having a higher tenacity have been eagerly desired and many attempts of developing high-tenacity fibers have heretofore been made. With regard to polyamide fibers, proposals of making high-tenacity polyamide fibers were made, for example, in Japanese Unexamined Patent Application No. 1-168913 and Japanese Unexamined Patent Publication No. 3-241007.
Namely, a high-tenacity polyhexamethylene adipamide fiber having a special structure defined by specific fiber structural properties is described in Japanese Unexamined Patent Application No. 1-168913. This fiber is characterized by the following features (a) through (f) as compared with conventional polyhexamethylene adipamide fibers:
(a) the crystal orientation function is the same or higher, PA1 (b) the amorphous orientation fuction is higher, PA1 (c) the long period in the direction of the fiber axis is the same, PA1 (d) the long period in the direction perpendicular to the fiber axis is larger, PA1 (e) the main dispersion temperature of a mechanical loss tangent curve as obtained by a dynamic viscoelastic measurement is lower, and PA1 (f) the DSC melting point as measured by a Zep method is higher and the perfection of crystal is higher. PA1 (a) the crystalline perfection index is larger than about 73, PA1 (b) the long-period interplanar spacing is larger than about 100 angstrom, PA1 (c) the long-period intensity (LPI) is larger than 1.0, PA1 (d) the apparent crystallite size (ACS) is larger than about 55 angstrom, PA1 (e) the density is larger than 1,143, PA1 (f) the birefringence is larger than about 0.06, PA1 (g) the differential birefringence (.DELTA..sub.90-00) is positive, and PA1 (h) the crystalline orientation angle is larger than 10.degree.. PA1 (a) the differential birefringence .delta..DELTA.n as defined by the following equation .delta..DELTA.n=.DELTA.n.sub.s -.DELTA.n.sub.c is in the range of -5.times.10.sup.-3 to 0.times.10.sup.-3, where .DELTA.n.sub.s is birefringence at a distance of 0.9 of the distance spanning from the center to the surface of the fiber, and PA1 (b) .DELTA.n.sub.c long period (Dm) in the direction of the fiber axis and the long period (De) in the direction perpendicular to the fiber axis satisfy the following formulae: PA1 (d) the birefringence (An) is: EQU .DELTA.n.gtoreq.60.times.10.sup.-3 PA1 (e) the crystal orientation function (fc) is: EQU fc.gtoreq.0.88, PA1 (f) the amorphous orientation function (fa) is: EQU fa=0.70 to 0.85 PA1 (i) 50 to 80% by weight, based on the total weight of the treating agent, of a diester compound, PA1 (ii) 0.3 to 10% by weight, based on the total weight of the treating agent, of a sodium salt of a phosphated product of an ethylene oxide-added (n=1 to 7) branched alcohol having 8 to 26 carbon atoms, and PA1 (iii) 10 to 40% by weight, based on the total weight of the treating agent, of a nonionic surfactant obtained by the reaction of an addition product of ethylene oxide to a polyhydric alcohol, with a monocarboxylic acid and a dicarboxylic acid. PA1 .DELTA. is birefringence, PA1 X is degree of crystallization, PA1 fc is crystal orientation function, PA1 fa is amorphous orientation function, PA1 .DELTA..degree.c is intrinsic birefringence of the crystalline region, PA1 A.degree.a is intrinsic birefringence of the amorphous region PA1 (both .DELTA..degree.c and .DELTA..degree.a are 0.73). PA1 1/0.2 A!/ B! (by weight) 1/4, PA1 1/0.2 A!/ B! (by weight) 1/3.
In other words, the high-tenacity polyhexamethylene adipamide fiber has a fiber structure capable of developing a high tenacity, i.e., features (a) and (b), as well as a fiber structure capable of developing stability against mechanical functions, i.e., features (d), (e) and (f). More practically, this fiber has a high-tenacity, a good dimensional stability to high temperature, a good tenacity-maintenance after vulcanization and a good fatigue resistance.
The above-mentioned high-tenacity polyhexamethylene adipamide fiber is made by a process characterized by the combination of a spinning at a high rate and a heat drawing at a relatively low rate. Namely, a spinning at a high rate is employed for developing the features (d), (e) and (f) and a heat drawing at a relatively low rate is employed for developing the features (a), (b) and (c). By a high speed spinning, a stable structure can be easily obtained but a high-tenacity structure is difficult to obtain. This problem is solved by combining the heat drawing at a relatively low rate with the high speed spinning in the process.
The above-mentioned high-tenacity polyhexamethylene adipamide fiber has a high tenacity, e.g., 12.5 g/d as specifically described in the working examples, but has a very low elongation, e.g., 12.0%. Further, the excellent toughness inherently possessed by a polyhexamethylene adipamide fiber is lowered in this fiber.
In Japanese Unexamined Patent Publication No. 3-241007, a polyamide fiber having a low shrinkage, a high modulus and a very high toughness, and a process for making the same are described. This polyamide fiber is characterized by the following structural features (a) through (h):
The polyamide fiber has a toughness of at least about 11.0 g/d, a dry heat shrinkage at 160.degree. C. of at least 6.5%, a modulus of at least about 35 g/d and a sound-wave modulus of at least 90 g/d.
This polyamide fiber is made by a process wherein a heat drawing is carried out under conditions such that the fiber temperature is at least 185.degree. C. and the residence time is about 0.05 to about 1 second, and then the heat-drawn fiber is subjected to a heat relaxation treatment under conditions such that the fiber temperature is at least 185.degree. C. and the residence time is specific. This process is characterized by a very long heat drawing time and a very long heat relaxation time, as compared with conventional processes for making polyamide fibers, especially a direct-spinning-drawing process which is recently a most typical process for making polyamide fibers.
More specifically, the above-mentioned polyamide fiber is made by a process wherein a completely drawn nylon 66 fiber is further subjected to drawing and heat-treatment in examples 1 to 4 and 6, or a process wherein an undrawn fiber is once wound up and then the fiber is subjected to a heat drawing and a heat-treatment. This process is not concerned with a direct spinning-drawing process wherein spinning, heat-drawing and heat-treatment are carried out in a completely continuous manner. This fact would be seen from the properties of the resulting nylon 66 fibers.
The nylon 66 fiber obtained by the process described in Japanese Unexamined Patent Publication No. 3-241007 has been subjected to a heat treatment under severe conditions and therefore is a high-tenacity fiber having a high density, a high crystalline completeness index and a high apparent crystallite size. However, the excellent toughness inherently possessed by a nylon 66 fiber is lowered in this nylon 66 fiber.
To impart a durability against the deterioration due to heat, light, oxygen and the other factors, antioxidants including copper compounds are incorporated in a nylon 66 fiber. The incorporated copper compounds are liable to be partially thermally decomposed in the polymerization step and the melt-spinning step, whereby part of the copper compounds are converted to compounds which are insoluble in the polymer, namely, converted to contaminative aggregate particles. It is important to uniformly disperse the copper compounds in the polymer (i.e., to avoid the formation of portions wherein the compounds are present in a high concentration) and to minimize the thermal history of the copper compounds for preventing the thermal decomposition of the antioxidants including the copper compounds.
It is advantageous in view of a uniform dispersion that copper compounds are incorporated in the polymerization step as conventionally carried out, but a problem arises in that contaminative aggregate particles are undesirably formed by the fact that the copper compounds are subject to thermal decomposition due to the large thermal history in the polymerization step. Where a master polymer in the form of chips having incorporated therein a salient amount of a copper compound is prepared and, immediately before the melt-spinning, the master polymer is incorporated with a polymer having not incorporated therein a copper compound, the master polymer containing the copper compound in a high concentration is heated in the pelletizing step whereby a salient amount of decomposed products of the copper compound are inevitably produced. Where a powdery copper compound is incorporated with polymer chips, it is difficult to uniformly disperse the copper compound or once-adhered copper compound is occasionally come off from the chips, portions containing the copper compound in a high concentration are formed in the resulting fiber.